A new dual-modality (SPECT/x-ray CT) three-dimensional imaging system to quantitatively map the distribution of tumor-specific radiolabeled molecules in small animal models and to register that distribution with high resolution anatomic image data is proposed. The proposed system will employ a new high-resolution, high sensitivity detector design and a new iterative SPECT reconstruction algorithm. The reconstruction algorithm will exploit the availability of a co-registered x-ray CT data set for attenuation correction. In the Phase I feasibility study, a prototype detector and multi-channel readout electronics will be developed and. incorporated into a bench-top prototype microSPECT scanner. The prototype microSPECT scanner will be used in conjunction with existing microCT scanners for dual-modality studies of test phantoms and mouse models. Quantitative microSPECT images will be compared with traditional pharmacokinetic data to evaluate the technology and guide the Phase II development. The ultimate goal of this Phase I/Phase II effort is to develop and commercialize an easy-to-use high-resolution dual-modality imaging system for preclinical development and evaluation of cancer imaging and therapy compounds.